Research On Microstructure Evolution And Mechanical Properties Of Low-temperature Bainitic Steel

Low-temeperature bainitic steel is one kind of high-performance structural steel with a unique combination of ultra-high strength,good ductulity and toughness.The increase of strength usually leads to the significant decrease of ductility and toughness for high strength steels.It is important to control the fraction and morphology of retained austenite and obtain fine bainitic microstructure by suitable heat-treatment method,which is helpful to achieve excellent mechanical properties and promote the practical applications of low-temeperature bainitic steel.Two kinds of high-carbon low alloy steels 0.63C-1.47Si-0.85Mn-1.53 Cr and 0.98C-2.95Si-0.77Mn-0.43 Cr are selected as the study samples in this paper.The microstructure evolution process and mechanical properties of high-carbon low alloy steels subjected to isothermal treatment and multiple step isothermal treatment are systematically investigated.The morphology,distribution and volume fraction of retained austenite as well as their effects on mechanical properties are discussed.The research results show that:(1)The microstructure of 0.63C-1.47Si-0.85Mn-1.53 Cr steel austempered at 240°C-300°C is composed of lath-like bainitic ferrite with the lath width of 60-160 nm and retained austenite.With the increase of austempering temperature,the content of retained austenite and the width of bainitic ferrite lath both increase.0.63C-1.47Si-0.85Mn-1.53 Cr steel austempered at 240-300°C exhibits ultra-high strength and hardness,1749-2200 MPa tensile strength,1264-1703 MPa yield strength,45.5-58 HRC hardness,4-22% elongation and 10-60.5J Charpy-U impact energy.With the increase of austempering temperature,both strength and hardness decrease,but the elongation and impact energy increase.With prolonging the austempering time after finishing bainite transformation,strength and hardness are little changed and the impact energy increases firstly,then decreases again.Samples austempered at 300°C for 9h achieve the best mechanical properties,strength 1749 MPa,yield strength 1376 MPa,total elongation 22%,product of strength and elongation 38.48 GPa%,hardness 45.5HRC and impact energy 60.5J.Austempered samples possess excellent perfoemance as a result of retained austenite,and it occurs strain-induced martensite under stress,thereby providing a superior TRIP effect.(2)0.98C-2.95Si-0.77Mn-0.43 Cr steel subejcted to austempering treatment exhibit ultra-high strength and hardness,1847-2294 MPa tensile strength,1332-1776 MPa yield strength,46.5-54 HRC hardness,2-23% elongation and 4.76-48 J Charpy-U impact energy.Effect of austempering time and temperature on mechanical properties is in basic agreement with that of 0.63C-1.47Si-0.85Mn-1.53 Cr steel.TRIP effect of retained austenite,nanotwins in austenite resulting in fine-grained strengthening,uniform distribution of graphite in microstructure are as possible explanations for affecting the mechanical properties of austempered samples in 0.98C-2.95Si-0.77Mn-0.43 Cr steel.(3)Banite sheaves in the microstructure of 0.63C-1.47Si-0.85Mn-1.53 Cr steel subjected to multiple step isothermal treatment is finer than that of samples austempered at 300°C.Meanwhile,the volume fraction of blocky retained austenite obviously decreases.Charpy-U impact energy of 68 J can be obtained by multiple step isothermal treatment and the heat treatment time is 1.5h.